It has been known for about two decades that high molecular weight polymers can be prepared by copolymerizing carbon dioxide with epoxy compounds in order to provide the corresponding poly(alkylene carbonates). These polymers exhibit unique chaining of the monomers by the alternate incorporation of carbon dioxide and epoxide moieties in the polymer chain. Because such high molecular weight polymers decompose cleanly, they find use in lost foam molding applications and as binders for ceramic or metallic particles in sintered molding procedures. The polymers can also be fabricated into films and other shaped articles and used in blends with other polymers for various applications such as adhesives. Tailoring the polymer to specific end uses, however, requires the molecular weight of the polymer to be controlled to a desired level and, in general, this has not been achievable without also decreasing the productivity of the catalyst employed.
The problem of molecular weight control occurs in many polymer systems. One of the proposed solutions to the problem is the addition of certain chain terminating agents which cut short the polymerization of the long chain molecule. For example, Baggett in U.S. Pat. Nos. 4,020,045 (1977) and 4,059,566 (1977) describes attempts to control the molecular weight of polycarbonates formed from phosgene and dihydric phenols by the addition of a metallic sulfite to the polymerization as a chain terminating agent, or the use of ammonia or ammonia compounds as chain terminators in the same polymerization. The polycarbonates formed from the reaction of phosgene and dihydric phenols result from a polycondensation reaction. The polycarbonates described in this invention are formed via an anionic coordination mechanism which is completely different in character from a polycondensation reaction. Therefore, the teachings of the Baggett reference do not apply to the process of this invention.
Soga, et al.. Makromol. Chem., 179, 2837-2343 (1978), discloses alternating copolymerization of carbon dioxide and epoxypropane in dioxane using a cobalt diacetate catalyst and acetic acid to decrease the number average molecular weight. Soga states that there is very little loss in yield of the polymer based on the catalyst and it is suggested that the acetic acid causes a chain transfer reaction. Other carboxylic acids, such as benzoic acid, chloroacetic acid, dichloroacetic acid and trichloroacetic acid were said to cause similar transfer reactions. The data presented, however, show that these other carboxylic acids produced significant reductions in yield. The reduction in yield when benzoic acid was used was over 90%. Also a small amount of epoxypropane homopolymer was produced in addition to the copolymer. Although a reference is made to acetic acid salts of chromium, zinc and nickel as other possible catalysts, no information is given on the effect of carboxylic acid in polymerizations using these catalysts.
Japanese Patent Application No. 55-12156, Ikeda, et al., Tokyo Institute of Technology (1980), discloses using cobalt acetate or diethyl zinc as a catalyst for copolymerizing carbon dioxide and an epoxy compound in the presence of various carboxylic acids; for example, acetic acid, benzoic acid, trichloroacetic acid, lactic acid and steric acid, in order to regulate the molecular weight. This appears to be based upon the same work reported by Soga, et al., cited above, in which the carboxylic acids other than acetic acid caused significant decreases in the productivity of the cobalt acetate catalyst.
Rokicki and Kuran, "The Application of Carbon Dioxide as a Direct Material for Polymer Synthesis in Polymerization and Polycondensation Reactions," J. Macromol. Sci.-Rev. Macromol. Chem., C21(1), 135-136 (1981), present a survey of scientific literature on the use of carbon dioxide in polymerization and polycondensation reactions and describe, inter alia, the copolymerization of carbon dioxide with oxiranes using organozinc catalysts, such as diethylzinc-pyrogallol and zinc carboxylates as well as metallo-organic catalysts of cobalt. chromium, nickel, magnesium and aluminum, thereby indicating that a relatively large number of catalysts are active in promoting the copolymerization between carbon dioxide and oxiranes. Catalysts based on diethylzinc predominate in reports on the alternate copolymerization of carbon dioxide and oxiranes, but coordination catalysts, for example, metal carboxylates, have been less widely studied. Zinc derivatives, however, are said to exhibit higher activity than derivatives of cobalt or cadmium, while derivatives of aluminum, magnesium, chromium and nickel lead to low molecular weight polymers. Some ability to control molecular weight is said to result from increasing the carbon dioxide pressure in order to increase number average molecular weight or increasing reaction temperature to decrease the molecular weight. An increase in reaction time to increase molecular weight is somewhat effective, although prolonged reaction periods may cause polymer degradation.
Soga, et al., Polymer Journal, 13, pages 407-410 (1981), discloses alternating copolymerization of carbon dioxide and propylene oxide with catalysts prepared from zinc hydroxide and dicarboxylic acids, but there is no suggestion of how the molecular weight of the polymers can be controlled when using this catalyst system.
Soga, Nippon Kagakkaishi, Vol. 2, 295-300 (1982), investigates several types of catalyst systems which promote alternate copolymerization of carbon dioxide and alkylene oxides as possible improvements over the known catalyst system of diethylzinc and water. These alternate catalysts include metal oxide-supported diethylzinc, acetic acid salts of cobalt and zinc, reaction products of zinc hydroxide and dicarboxylic acids and metal oxide-supported zinc, cobalt and aluminum halides. The zinc dicarboxylate formed by reacting zinc hydroxide with glutaric acid was said to be about 3 times as active as the diethyl zinc .cndot.H.sub.2 O system, but the catalyst prepared by reacting zinc oxide with glutaric acid was said to offer no improvement at all over the diethylzinc .cndot.H.sub.2 O system. Molecular weight of the polymers produced varies considerably with the choice of catalyst, and the only attempt to control the molecular weight for one specific catalyst system was with the use of acetic acid added to the catalyst system of cobalt acetate. It was reported that the yield of the polymer with respect to catalyst did not decrease, but the number average molecular weight of the polymer decreased in proportion to the acetic acid added.
Aida, et al. Macromolecules, 19, pages 8-13 (1986), discloses that when using a catalyst of aluminum porphyrin-triphenylphosphine to copolymerize carbon dioxide and epoxides, the molecular weight of the polymer can be regulated by the monomer-to-catalyst ratio. This, however, is typical for "living" catalyst systems. The same molecular weight control mechanism does not apply to zinc-based systems such as the zinc dicarboxylate described above. It is stated that although a number of catalyst systems are known, no other successful method of controlling molecular weight of the copolymer has been reported.